Quantifying Hot Electron Energy Contributions in Plasmonic Photocatalysis Using Electrochemical Surface-Enhanced Raman Spectroscopy

Quantifying Localized Surface Plasmon Resonance Induced Enhancement on Metal@Cu2O Composites for Photoelectrochemical Water Splitting

The localized surface plasmon resonance (LSPR) of metal nanoparticles can substantially enhance the activity of photoelectrocatalytic (PEC) reactions. However, quantifying the respective contributions of different LSPR mechanisms to the enhancement of PEC performance remains an urgent challenge. In this work, Cu@Cu2O composites prepared by annealing Cu2O under an inert atmosphere and electrodeposited metal@Cu2O composites (MED@Cu2O, MED = CuED, AuED, AgED, PdED, PtED) are employed as platform materials to investigate the LSPR effect on the PEC hydrogen evolution reaction (HER). All the composites exhibited remarkably LSPR-enhanced activity toward PEC HER. The contributions of two LSPR mechanisms, plasmon induced resonance energy transfer (PIRET) and hot electron transfer (HET), to the photocurrent on Cu@Cu2O and CuED@Cu2O are quantified by using different bands of incident light. Moreover, using MED@Cu2O composites, the effects of both the metal species and the applied potential on HET are quantitatively investigated. The results reveal that a pronounced HET enhancement occurs only when the LSPR peak energy is lower than the semiconductor bandgap energy (Eg) and that HET strengthens as the applied potential becomes more negative for PEC HER. This work therefore provides a quantitative understanding of the roles of PIRET and HET in boosting PEC activity.

Plasmon-Driven Chemistry

Plasmonic nanomaterials are promising photocatalysts due to their large optical cross sections and facile generation of nanoscale hotspot regions. They have been used to drive a range of photochemical reactions, including H2 dissociation, CO2 reduction, and ammonia synthesis, offering an exciting approach to light-driven chemistry. Deepening our understanding of how energy can be controllably transferred from the plasmonic nanomaterial to proximal reactants should lead to improvements in the efficiency and selectivity in plasmonic photocatalysis. Here we provide a comprehensive overview of plasmonic properties and explore different energy partitioning pathways. We focus on the importance of mapping molecular potential energy landscapes to understand reactivity and describe recent advancements in spectroscopic techniques, such as ultrafast surface-enhanced Raman spectroscopy, electron microscopy, and electrochemistry, that can aid in understanding how plasmonic nanomaterials can be used to shape potential energy surfaces and modify chemical outcomes. Additionally, we explore innovative hybrid plasmonic nanostructures such as antenna-reactor complexes, plasmonic single-atom catalysts, plasmonic picocavities, and chiral plasmonic substrates, all of which show great promise in advancing the field of plasmon-driven chemistry.

Monitoring Hot Holes in Plasmonic Catalysis on Silver Nanoparticles by Using an Ion Label

Energetic carriers generated by localized surface plasmon resonance (LSPR) provide an efficient way to drive chemical reactions. However, their dynamics and impact on surface reactions remain unknown due to the challenge in observing hot holes. This makes it difficult to correlate the reduction and oxidation half-reactions involving hot electrons and holes, respectively. Here we detect hot holes in their chemical form, Ag(I), on a Ag surface using surface-enhanced Raman scattering (SERS) of SO32- as a hole-specific label. It allows us to determine the dynamic correlations of hot electrons and holes. We find that the equilibrium of holes is the key factor of the surface chemistry, and the wavelength-dependent plasmonic chemical anode refilling (PCAR) effect plays an important role, in addition to the LSPR, in promoting the electron transfer. This method paves the way for visualizing hot holes with nanoscale spatial resolution toward the rational design of a plasmonic catalytic platform.

Raman Monitoring of the Electro-Optical Synergy-Induced Enhancements in Carbon-Bromine Bond Cleavage, Reaction Rate, and Product Selectivity of p-Bromothiophenol

Electro-optical synergy has recently been targeted to improve the separation of hot carriers and thereby further improve the efficiency of plasmon-mediated chemical reactions (PMCRs). However, the electro-optical synergy in PMCRs needs to be more deeply understood, and its contribution to bond dissociation and product selectivity needs to be clarified. Herein, the electro-optical synergy in plasmon-mediated reduction of p-bromothiophenol (PBTP) was studied on a plasmonic nanostructured silver electrode using in situ Raman spectroscopy and theoretical calculations. It was found that the electro-optical synergy-induced enhancements in the cleavage of carbon-bromine bonds, reaction rate, and product selectivity (4,4'-biphenyl dithiol vs thiophenol) were largely affected by the applied bias, laser wavelength, and laser power. The theoretical simulation further clarified that the strong electro-optical synergy is attributed to the matching of energy band diagrams of the plasmonic silver with those of the adsorbed PBTP molecules. A deep understanding of the electro-optical synergy in PBTP reduction and the clarification of the mechanism will be highly beneficial for the development of other highly efficient PMCRs.

Determining Quasi-Equilibrium Electron and Hole Distributions of Plasmonic Photocatalysts Using Photomodulated X-ray Absorption Spectroscopy

Most photocatalytic and photovoltaic devices operate under broadband, constant illumination. Electron and hole dynamics in these devices, however, are usually measured by using ultrafast pulsed lasers in a narrow wavelength range. In this work, we use excited-state X-ray theory originally developed for transient X-ray experiments to study steady-state photomodulated X-ray spectra. We use this method to attempt to extract electron and hole distributions from spectra collected at a nontime-resolved synchrotron beamline. A set of plasmonic metal core-shell nanoparticles is designed as the control experiment because they can systematically isolate photothermal, hot electron, and thermalized electron-hole pairs in a TiO2 shell. Steady-state changes in the Ti L2,3 edge are measured with and without continuous-wave illumination of the nanoparticle's localized surface plasmon resonance. The results suggest that within error the quasi-equilibrium carrier distribution can be determined even from relatively noisy data with mixed excited-state phenomena. Just as importantly, the theoretical analysis of noisy data is used to provide guidelines for the beamline development of photomodulated steady-state spectroscopy.

Quantifying the ultrafast and steady-state molecular reduction potential of a plasmonic photocatalyst

Plasmonic materials are promising photocatalysts as they are well suited to convert light into hot carriers and heat. Hot electron transfer is suggested as the driving force in many plasmon-driven reactions. However, to date, there are no direct molecular measures of the rate and yield of plasmon-to-molecule electron transfer or energy of these electrons on the timescale of plasmon decay. Here, we use ultrafast and spectroelectrochemical surface-enhanced Raman spectroscopy to quantify electron transfer from a plasmonic substrate to adsorbed methyl viologen molecules. We observe a reduction yield of 2.4 to 3.5% on the picosecond timescale, with plasmon-induced potentials ranging from [Formula: see text]3.1 to [Formula: see text]4.5 mV. Excitingly, some of these reduced species are stabilized and persist for tens of minutes. This work provides concrete metrics toward optimizing material-molecule interactions for efficient plasmon-driven photocatalysis.

Plant Extract-Based Fabrication of Silver Nanoparticles and Their Effective Role in Antibacterial, Anticancer, and Water Treatment Applications

Ammi visnaga is a biennial or annual herbaceous plant belonging to the family Apiaceae. For the first time, silver nanoparticles were synthesized using an extract of this plant. Biofilms are a rich source of many pathogenic organisms and, thus, can be the genesis of various disease outbreaks. In addition, the treatment of cancer is still a critical drawback for mankind. The primary purpose of this research work was to comparatively analyze antibiofilms against Staphylococcus aureus, photocatalytic activity against Eosin Y, and in vitro anticancer activity against the HeLa cell line of silver nanoparticles and Ammi visnaga plant extract. The systematic characterization of synthesized nanoparticles was carried out using UV-Visible spectroscopy (UV-Vis), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), atomic force microscopy (AFM), dynamic light scattering (DLS), zeta potential, and X-ray diffraction microscopy (XRD). The initial characterization was performed with UV-Vis spectroscopy, where a peak appeared at 435 nm, which indicated the SPR band of the silver nanoparticles. AFM and SEM were performed to determine the morphology and shape of the nanoparticles, while EDX confirmed the presence of Ag in the spectra. The crystalline character of the silver nanoparticles was concluded with XRD. The synthesized nanoparticles were then subjected to biological activities. The antibacterial activity was evaluated by determining the inhibition of the initial biofilm formation with Staphylococcus aureus using a crystal violet assay. The response of the AgNPs against cellular growth and biofilm formation was found to be dose dependent. Green-synthesized nanoparticles showed 99% inhibition against biofilm and bacteria, performed excellent anticancer assay with an IC₅₀ concentration of 17.1 ± 0.6 µg/mL and 100% inhibition, and photodegradation of the toxic organic dye Eosin Y up to 50%. Moreover, the effect of the pH and dosage of the photocatalyst was also measured to optimize the reaction conditions and maximum photocatalytic potential. Therefore, synthesized silver nanoparticles can be used in the treatment of wastewater contaminated with toxic dyes, pathogenic biofilms, and the treatment of cancer cell lines.